A power converter dissipates a relatively large amount of power upon transistor opening and closing during power cycles. If the power being dissipated is too large, semiconductor devices may over-heat, or otherwise become stressed, and become damaged. One mechanism to reduce the need to dissipate such power is to switch the transistors at intervals where the power would be the smallest, sometimes referred to as zero voltage switching (ZVS), or zero current switching (ZCS).
“High-power” power converters often use insulated-gate bipolar transistor (IGBT) modules, which have relatively large voltage ratings, but are typically limited to switching frequencies of less than two kilohertz (kHz). IGBTs also have relatively large voltage drops, and are thus inherently inefficient. Because IGBTs are unidirectional, IGBTs are relatively immune to oscillations.
Metal oxide semiconductor field-effect transistors (MOSFETs) facilitate relatively high switching frequencies and relatively high power densities. For example, silicon carbide (SiC) MOSFETs have voltage ratings of up to 1200 volts (V), and gallium nitride (GaN) MOSFETs have voltage ratings of up to 600V. MOSFETs also facilitate switching frequencies in excess of 100 kHz, and are relatively efficient because the voltage drop is primarily limited only by Drain-to-Source On Resistance (RDSon).
Higher switching frequencies reduce the timing window for ZVS. Accordingly, the drive signals that drive the gate of the MOSFET must be relatively fast, and the timing relatively accurate. MOSFETs are bidirectional devices, such that when a MOSFET is in an on state it exhibits a relatively low impedance in each direction through the device. Accordingly, circuits that use MOSFETs are susceptible to oscillations (sometimes referred to as “ringing”) as charge can move in both directions through the device.
Some power supplies rely on “floating magnetics.” Moreover, for safety concerns, it is relatively common practice to galvanically isolate high-voltage power supplies. A galvanically-isolated power supply has a higher possibility of developing a common-mode voltage offset due to switching parasitics, resulting in undesirable oscillations, which may lead to device failure.